Fix six with metaclass

[hippo91]

Steps

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Description

This PR corrects the brain dealing with six.with_metaclass.
The error came from the tweaking of the node bases. It has not to be done.
For example, following code:

from astroid import extract_node
src = """
class A:
    pass

def get_parent():
    return A

class B(get_parent()):
    pass

B
"""
node=extract_node(src)
cls_node = node.inferred()[0]
print(list(cls_node.bases))
print(list(cls_node.ancestors()))

gives:

[<Call l.8 at 0x7f0ad5ecc250>]
[<ClassDef.A l.2 at 0x7f0ad5faf350>, <ClassDef.object l.0 at 0x7f0ad6ea45d0>]

The base is a CallNode, whereas the ancestors are ClassDef from the class hierarchy.
Thus in the six.with_metaclass case, the base has to be a call node.
Doing this, the six module is seen as really used and this avoids the emission of unused-import message.

This PR should correct the failing CI on pylint side.

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Failing CI on pylint.

[hippo91]

@fmigneault are you ok with this change?
Related discussion:
#890 #841 pylint-dev/pylint#4006